When a compact is obtained from a powder material by compressing, sintering or other processes, a powder-filling system for putting powder into a container (shaping container) designed for molding (shaping) the powder is used. In such a powder-filling system, the container must be uniformly filled with powder at a predetermined density. Furthermore, in many cases, the filling density of the powder is required to be higher than the level achieved by simply pouring the powder into the container (this is called the “natural filling”). The operation of filling the container at a higher density than the density achieved by the natural filling is hereinafter called the “dense filling.”
As one example of the system for the dense filling, Patent Literature 1 discloses a system which employs the air-tapping method to fill a container with powder. In this system, a hopper having an opening in its lower portion is attached to a powder-filling container in a removable and hermetically closable fashion so that the hopper communicates with the container at the opening. The system also has a powder supplier for supplying powder to the hopper and a gas supplier for supplying compressed gas to the hopper. As the compressed gas, air can be used if the filling powder is a hard-to-oxidize powder. If the filling powder is an easy-to-oxidize powder, inert gas should be used, such as nitrogen or argon gas.
At the opening in the lower portion of the hopper, a planer sieve member having a sieve with a predetermined size of openings is provided. The sieve may consist of a grid mesh, parallel wires (a set of parallel wires arranged with predetermined spacing), perforated plate (a thin plate with a number of punched holes) or the like. The size of the openings of the sieve is adjusted so that the powder to be supplied to the container as a whole will not fall naturally but will fall when pressure is applied by compressed gas in a manner to be described later. Needless to say, the size of the openings of the sieve should be greater than the size of the individual particles forming the powder (which are hereinafter called “powder particles”). If the powder particles are highly cohesive, the size of the openings of the sieve needs to be much greater than the powder particles, since the problem in this situation is to control the passage of aggregates of powder particles rather than individual powder particles. The degree of cohesion of the powder particles depends on the electric charges (static electricity) and magnetism possessed by the powder particles or wetness on the surface of the powder particles, the shape of the powder particles, and other factors. In general, finer powder particles have a higher degree of cohesion.
The powder-filling system of Patent Literature 1 is used as follows: Initially, an amount of powder is supplied from the powder supplier to the hopper. At this stage, the powder does not fall off the hopper, since the size of the openings of the sieve is adjusted in the previously described manner. Next, the hopper is attached to the container and hermetically closed. Subsequently, compressed gas is rapidly charged through a gas introduction port into the space above the powder within the hopper, and after a short period of time, the compressed gas is discharged from the hopper. Such a charge and discharge of the compressed gas is alternately repeated at a frequency of several tens of times per second (several tens of Hz), to repeatedly apply pulsed pressures to the top face of the powder within the hopper by the compressed gas. This operation makes the powder gradually pass through the sieve member and fall into the container. After a sufficient amount of powder is supplied to the container, with the top face of the powder above the sieve member, the hopper is removed from the container. This separates the powder held in the container from the powder remaining in the hopper, with the sieve member as the boundary.